1. The Field of the Invention
The present invention relates to the manufacture of Fisher-Tropsch catalysts for making synthetic hydrocarbon fuels from carbon monoxide and hydrogen. More particularly, the invention relates to skeletal iron catalysts that incorporate a promoter metal to increase the yield of C5+ hydrocarbons in Fischer-Tropsch synthesis reactions.
2. Related Technology
Skeletal catalysts, also known as “sponge-metal catalysts”, have been known for decades and are used extensively in many industries and organic synthesis. The use of skeletal catalysts is particularly prevalent in reactions involving hydrogenation, dehalogenation, and desulfurization. The first skeletal catalysts were developed by Murrey Raney in the 1920's using aluminum-nickel alloys. The catalyst is prepared by treating a block of nickel-aluminum alloy with sodium hydroxide to remove a portion of the aluminum. Removing a portion of the aluminum leaves behind a porous nickel framework. The porous framework gives the catalyst high surface area and increased catalytic activity. These nickel-based skeletal catalysts are often called “Raney Nickel.” More recently, iron-based skeletal catalysts have been developed by Hydrocarbon Technologies, Inc. for use in Fischer-Tropsch synthesis reactions for making synthetic fuels.
Technologies for making synthetic fuels typically begin with the partial oxidation of carbon-based materials such as methane or coal to produce carbon monoxide and hydrogen, commonly known as “synthesis gas” or “syngas”. Next, in a Fischer-Tropsch reaction, the synthesis gas is converted to more valuable hydrocarbons such as naphtha, diesel and paraffin wax. The general chemistry of the Fischer-Tropsch reaction is as follows:nCO+(2n+1)H2→CnH2n+2+nH2O  (1)CO+H2O→CO2+H2  (2)2nCO+(n+1)H2→CnH2n+2+nCO2  (3)
One competing reaction may be the water-gas shift reaction, equation (2), in which carbon monoxide is consumed in a reaction with water generated from equation (1), above, to form carbon dioxide (CO2) and hydrogen (H2). The net effect is the consumption of at least some of the water produced in equation (1) and an alteration in the H2:CO ratio.
The Fischer-Tropsch reaction is carried out in the presence of a catalyst. Catalysts used in the Fischer-Tropsch process vary in composition based upon the product mixture desired and reaction conditions employed but commonly comprise at least one catalytic metal selected from Group VIIIA, preferably Co, Ru, Fe or Ni. Iron is frequently used because of its high reactivity and lower cost than other suitable metals. Furthermore, iron catalysts have high water-gas shift activity and tend to favor reaction (3) above.
Methods for making iron-based Fischer-Tropsch catalysts are known in the art. Two of the most common iron-based Fischer-Tropsch catalysts are precipitated iron and fused iron. Preparation of a precipitated iron catalyst typically consists of precipitating iron hydroxides and oxides from an aqueous solution; washing, drying and calcining the precipitate; and pretreating the catalyst.
Fused iron catalysts are typically prepared by adding promoters to the melted oxide at high temperature. Solid chunks are obtained from the cooled mixture, then ground and sized. The specific catalytic activity of fused iron catalysts is generally lower than that of precipitated iron catalysts. The catalytic activity of fused iron has been measured in stirred-tank reactors as half that of precipitated iron catalysts (See Fuel Processing Technology, 1992, Vol. 30, pp. 83-107).
While precipitated iron catalysts have greater activity than fused iron catalysts, precipitated iron catalyst have their own disadvantages. For example, precipitated iron is typically in the form of very fine metal particles. These small particles can escape from the reactor as a contaminant of the hydrocarbon product, especially waxy products. Iron particulates contaminating waxy products are often very difficult to remove from waxy products, thereby significantly increasing the processing costs of these products and diminishing the value of the overall process.
Recently, skeletal iron catalysts have been developed for use in Fischer-Tropsch reactions. Skeletal iron catalysts advantageously have high surface areas, and therefore high activity, but are less likely to contaminate the waxy product as compared to precipitated iron catalysts. The skeleton iron catalyst has also proven to be a good potential Fischer-Tropsch catalyst because it needs no pretreatment prior to test, has lower cost per unit mass of metal, good resistance to poisoning, and good structural uniformity.
Despite the advantages of a skeletal iron catalyst, there is still a need to improve the skeletal iron catalyst such that it can better compete economically with traditional sources of hydrocarbons fuels. One inefficiency with all Fischer-Tropsch catalysts is chain termination that results in production of low value short chain hydrocarbons (e.g., C2-C4 hydrocarbons). Chain termination can occur when a hydrogen radical reacts with the growing end of the hydrocarbon chain. Once chain termination has occurred, the length of the hydrocarbon cannot be extended. While it is desirable for chain termination to eventually occur, it is generally undesirable for chain termination to produce C2-C4 hydrocarbons.